The present invention relates to methods and compositions for increasing the viscosity of a treatment fluid, and, more specifically, to methods and compositions for treating a subterranean formation using a viscosified treatment fluid that contains a multifunctional boronic acid crosslinking agent.
Many industrial applications, including those in the upstream energy industry, utilize viscosified fluids or “viscosified treatment fluids.” As used herein, the terms “treatment” or “treating” refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. The terms “treatment” and “treating,” as used herein, do not imply any particular action by the fluid or any particular component thereof. Treatment fluids can include, for example, drilling fluids, fracturing fluids, gravel packing fluids, acidizing fluids, conformance treatment fluids, damage control fluids, remediation fluids, scale removal and inhibition fluids, chemical floods, and the like. Generally, viscosified treatment fluids that are used in subterranean operations are aqueous-based fluids that comprise gelling agents. These gelling agents can be biopolymers or synthetic polymers. Common gelling agents that can be used in viscosified treatment fluids can include, for example, galactomannan gums, cellulosic polymers, and polysaccharides.
Most viscosified treatment fluids crosslink the gelling agent using a crosslinking agent to increase the fluid's viscosity. Common crosslinking agents can comprise a metal ion, a transition metal, or a metalloid, which are collectively referred to herein as “metal(s).” Illustrative metals suitable for crosslinking can include, for example, aluminum, antimony, zirconium, magnesium and titanium. Generally, the metal of a crosslinking agent can interact with at least two gelling agent molecules to form a crosslink between them, thereby forming a crosslinked gelling agent.
Although conventional metal crosslinking agents can frequently be used in viscosified treatment fluids, the use of such crosslinking agents can be problematic because they may not form a viscoelastic gel below a critical concentration of gelling agent (e.g., the critical overlap concentration C*). In addition, such viscosified treatment fluids may not be thermally stable at high temperatures (e.g., temperatures exceeding about 300° F.), such that a loss of viscosity occurs over time. To offset these types of viscosity losses, the concentration of the gelling agent and/or the crosslinking agent can be increased, albeit at an increased cost of goods. Further, higher concentrations of the gelling agent and/or the crosslinking agent can make the viscosified treatment fluid more difficult to remove from a subterranean formation.
Various crosslinking agents have been investigated that are not based upon metals. For example, acrylamide-containing polymers, copolymers, and partially hydrolyzed variants thereof can be gelled with polyalkyleneimine and polyalkylenepolyamine crosslinking agents. In addition, boron-containing compounds such as, for example, boric acid and ulexite mineral (hydrated sodium calcium borate hydroxide) have been investigated as crosslinking agents with guar and other polysaccharide gelling agents. Boronic acid crosslinking agents, particularly boronic acid-containing polymers, have also been investigated in this regard. Boronic acid crosslinking agents can present particular advantages such as, for example, being able to crosslink a fluid at or near a neutral pH. At high pH values (e.g., >11), certain ions such as, for example, calcium and magnesium ions can precipitate and potentially damage a subterranean formation.
Although boronic acid-containing polymers can be at least comparable to metal crosslinking agents in many regards, boronic acid copolymers synthesized by conventional synthetic techniques can result in inefficient use of the boronic acid monomer units therein. Specifically, conventional techniques for synthesizing boronic acid copolymers can form gradient copolymers in which the boronic acid monomer units are clustered in less than the whole length of the polymer chain. Without being bound by theory or mechanism, the formation of gradient copolymers can result from different reaction rates of the monomers being used to form the copolymer. As a result of gradient copolymer formation, only a portion of the polymer chain can be available for crosslinking, which results in a sub-optimal use of material goods. Gradient copolymer formation can sometimes be addressed by using an excess of one or more monomers to drive the polymerization reaction kinetics toward a non-gradient copolymer product, but this approach again results in a sub-optimal use of material goods.
Accordingly, if boronic acid copolymers could be synthesized such that gradient copolymer formation is eliminated, minimized or reduced relative to conventional synthetic techniques, lower concentrations of these crosslinking agents could be utilized in a treatment fluid to obtain comparable crosslinking effects. In addition, such boronic acid copolymers might also allow lower concentrations of the gelling agent to be used in a treatment fluid, while still efficiently forming a gel.